FIG. 8 is a schematic cross-sectional view of conventional elastic wave device 1. Elastic wave device 1 includes piezoelectric substrate 2, interdigital transducer (IDT) electrode 3 disposed on piezoelectric substrate 2, wiring electrode 4 disposed on piezoelectric substrate 2 and connected to IDT electrode 3, sidewall 5 disposed on piezoelectric substrate 2 to surround the circumference of IDT electrode 3, top plate 7 placed on sidewall 5 to cover space 6 above IDT electrode 3, insulator 8 covering sidewall 5 and top plate 7, inductor electrode 9 disposed on insulator 8, insulator 10 covering inductor electrode 9 and an upper surface of insulator 8, terminal electrode 11 disposed on insulator 10, and connecting electrode 12 passing through insulators 8 and 10 to electrically connect wiring electrode 4, inductor electrode 9, and terminal electrode 11.
An elastic wave device similar to conventional elastic wave device 1 is disclosed in Patent Literature 1.
The upper surface of insulator 8 is ground to have a constant height and to expose a surface of connecting electrode 12. Inductor electrode 9 is then formed on this surface to electrically connect connecting electrode 12 to inductor electrode 9.
Conventional elastic wave device 1 may decrease a Q-factor and an inductance of inductor electrode 9.
FIG. 9 is a schematic cross-sectional view of another conventional elastic wave device 101. Elastic wave device 101 includes piezoelectric substrate 102, IDT electrode 103 disposed on piezoelectric substrate 102, wiring electrode 104 disposed on piezoelectric substrate 102 and connected to IDT electrode 103, sidewall 105 disposed on piezoelectric substrate 102 to surround IDT electrode 103, top plate 107 placed on sidewall 105 to cover excitation space 106 above IDT electrode 103, insulator 108 covering sidewall 105 and top plate 107, terminal electrode 109 disposed on insulator 108, and connecting electrode 110 passing through insulator 108 to electrically connect wiring electrode 104 to terminal electrode 109.
Elastic wave device 101 may necessarily include an inductor having a certain inductance and a certain Q-factor in order to improve characteristics, such as an insertion loss, an out-of-band attenuation, and isolation. It is difficult to form a conductive trace functioning as the inductor having such characteristics on piezoelectric substrate 102. Elastic wave device 101 may be necessary to add a separate inductor besides elastic wave device 101 for composing an electronic apparatus.